October 19, 2012

Acidification Changing Ocean Acoustics To Resemble Cretaceous Period

Along with negatively impacting marine life and global climate change, the acidification of the Earth´s oceans could have the unintended side effect of changing the acoustics beneath the water´s surface.

New research suggests that a future, more acidic ocean would resemble the one that existed around 110 million years ago, when dinosaurs still roamed both the land and sea. Climatologists have been raising concerns in recent years over the potential acidification of the world´s oceans, which is caused by the absorption of carbon dioxide into seawater.

Acoustics researchers David Browning and Peter Scheifele presented evidence to support this theory at the 164th meeting of the Acoustical Society of America in New Mexico. They said global warming-induced ocean acidification will allow low frequency, sub-200 Hz sounds to travel twice as far as they do in the current ocean.

The mechanism that would drive this change is a shift in the low frequency absorbing boron reaction that occurs in the ocean and is pH-dependent. As pH increases, the boron reaction and absorption of low frequency sound waves decreases–allowing those sounds to travel further, according to Browning and Scheifele.

"We call it the Cretaceous acoustic effect, because ocean acidification forced by global warming appears to be leading us back to the similar ocean acoustic conditions as those that existed 110 million years ago, during the Age of Dinosaurs,” said Browning, an acoustics professor at the University of Rhode Island.

The latest study builds on previous research that analyzed the boron levels in seafloor sediments to track ocean acidity over the past 300 million years.

Knowing the effects that boron has on underwater acoustics, Browning and Scheifele were able to model the oceans of 300 million years ago, during the Paleozoic Era. They found that low frequency sound transmission in the oceans was likely similar to what it is today. They also discovered that transmission of these frequencies improved as the seawater became more acidic–reaching the optimal transmission conditions about 110 million years ago when these sub-bass sounds traveled twice as far as they do today.

"This knowledge is important in many ways. It impacts the design and performance prediction of sonar systems," Browning said. "It affects estimation of low frequency ambient noise levels in the ocean. And it's something we have to consider to improve our understanding of the sound environment of marine mammals and the effects of human activity on that environment."

The changing oceans could have a significant impact on whales that use low frequency sounds to communicate between individuals. Humpback whales are the ocean´s most prolific singers and they employ sounds between 20 and 24 Hz in the baselines of their songs. These whales typically sing a phrase of 8 to 12 notes that is repeated for two to four minutes–in what is known as a theme. A collection of these themes is known as a song. Songs are usually around 30 minutes long and are often repeated for days. They can be heard for thousands of miles.